In order to construct a liquid crystal display unit excellent in image quality, that is, excellent in contrast ratio, light passing through a liquid crystal panel is required to be collimated as much as possible. According to the fruit of researches in recent years, for example, in order to obtain a contrast ratio not lower than 200 to 1, it is necessary that the angle of divergence of light passing through a liquid crystal panel is limited to a range of about 0.15 rad p--p in a first direction (in a direction of narrower directivity) and to a range of about 0.3 rad p--p (about twice as much as the aforementioned 0.15 rad p--p) in a second direction (in a direction of wider directivity).
A typical example of light collimating or collimator means in the prior art is a parabola mirror. A projection type liquid crystal display unit in the prior art is shown in FIG. 1.
In FIG. 1, the reference numeral 1 designates a light source; 2, a parabola mirror; 3, a liquid crystal panel; 4, a projection lens; and 5, a screen. The arrows show paths of light rays. The prior art has at least the following problems.
(1) In FIG. 1, light rays 6, 6' which reach the liquid crystal panel 3 directly without reflection by the parabola mirror 2 are not collimated. Accordingly, the contrast ratio and image quality of reproduced images are deteriorated. PA0 (2) In FIG. 1, the improvement of the efficiency of use of light and the improvement of the relative corner illuminance ratio are contradictory to each other. That is, when one is improved, the other is deteriorated. The term "relative corner illuminance ratio" used herein means the ratio of corner illuminance to central illuminance on the liquid crystal panel and is hereinafter abbreviated to "RCI (relative corner illuminance)". PA0 (3) In FIG. 1, the parabola mirror 2 is rotationally symmetrical with respect to the optical axis thereof. Accordingly, the sectional area of output light is shaped like a circle. When the radius of the circle is 1, the area is .pi.. On the other hand, the liquid crystal panel 3 is shaped like a rectangular inclusive of a square. The area of the rectangular inscribed in a unit circle is not larger than 2. Accordingly, a loss of about 36%(1-2/.pi.) occurs in the peripheral region because of mismatched aspect. PA0 (4) Because the light source 1 is surrounded by the parabola mirror 2 so that the path of an air flow is not linear, it is difficult to improve the efficiency of heat dissipation from the light source.
The result of inventor's analysis on the cause of the aforementioned problems on the basis of phisics or natural science will be described below with reference to FIGS. 1 and 2. FIG. 2 shows a coordinate system in which Z is taken in the direction of the optical axis of the parabola mirror 2 and r is the distance from the optical axis. Assume that the shape of the parabola mirror 2 is given by the following expression: EQU Z=0.5r.sup.2 /R.sub.1 (1)
in which R.sub.1 is the radius of curvature in the central region of the parabola mirror.
The light source 1 is located at the focal point (Z=0.5R.sub.1) of the mirror 2. Accordingly, output light reflected by the mirror becomes collimated light. Upon the assumption that the light source is isotropic, light intensity thereof is denoted by I [cd]. Accordingly, total light flux is 4 .pi.I[lm]. The increase of output light flux from the isotropic light source is proportional to the increase of the cosine of a zenith angle .theta. measured from the optical axis passing through the light source. The total light flux T collimated by the parabola mirror 2 and the efficiency of use of light E(.theta..sub.M) can be obtained as the following expressions. Incidentally, the aspect mismatch loss in the aforementioned item (3) is regarded to be neglected upon the assumption that the liquid crystal panel is shaped like a disk. ##EQU1##
On the other hand, the illuminance J of output collimated light of the parabola mirror 2 is inversely proportional to the square of the distance from the light source to the mirror. Accordingly, the following expression is obtained. ##EQU2##
The aforementioned expression means that the distance from every point on the mirror to the light source is equal to Z+0.5R.sub.1. When a value normalized by dividing output illuminance at every point on the mirror by illuminance in the central region of the mirror is made J.sub.1, the following expression is obtained. In the aforementioned expression, the parentheses () under the equal sign in the transforming process shows that an expression designated by the number put in the parentheses () is used for deducing the equal sign. This rule applies to succeeding expressions. ##EQU3##
Next, think of the expression of J.sub.1 in the zenith angle .theta.. The following expressions are obtained by using the aforementioned relation in which the distance between every point on the mirror and the light source in FIG. 2 is equal to Z+0.5R.sub.1. ##EQU4##
The expressions (3) and (8) are shown in FIGS. 3 and 4 respectively. It is apparent from FIG. 3 that the efficiency of use of light becomes 50% when .theta..sub.M is 0.5 .pi., that is, a right angle. Moreover, the efficiency of use of light becomes 75% when .theta..sub.M is 2 .pi./3. It is apparent from FIG. 4 that the relative corner illuminance becomes 25% when .theta. is 0.5 .pi.. In addition, the relative corner illuminance takes such a small value of 6.3% when .theta. is 2 .pi./3.
Although the aforementioned relations i.e. the expressions (4) to (8) have been obtained analytically, the relations may be also obtained on the basis of parabolic geometry alternatively. This is shown in FIG. 5. In FIG. 5, the dotted line 2' is the directrix of a parabola. The detailed description of FIG. 5 will be omitted.
As is understood from FIGS. 3 and 4, the prior art has a problem that the relative corner illuminance is deteriorated when the efficiency of use of light is improved.
As known commonly, a conventional one-panel type color liquid crystal display unit uses pigments of three primary colors for three-primary-color pixels. Accordingly, only energy not larger than one third the light energy generated by a white light source can be used (so-called shadow mask loss). As a measure to compensate for the shadow mask loss to thereby increase the efficiency of use of light by three times, provision of a three-direction means (which converts three primary colors into three different directions) and a microlens means on the light input side of the liquid crystal panel has been proposed in U.S. Pat. No. 5,161,042. In the aforementioned proposal, however, there is a problem that the angle of divergence of input light is degraded to about six times because the parallelism of light inputted to the liquid crystal panel is spoiled. The quality of reproduced images, that is, the contrast ratio of reproduced images is degraded so as to be nearly inversely proportional to the square of the divergence angle of the input light. Accordingly, the aforementioned proposal degrades the contrast ratio by about 36 times. Accordingly, the aforementioned proposal has not been put into practice yet.
Further, provision of lenticular lenses on the input side and output side of the liquid crystal panel has been proposed in JP-A-6-250177 of the present inventor. The proposal is, however, useless for the solution to the aforementioned contrast ratio degradation problem.
Further, in the projection type liquid crystal display unit in the prior art, there is a problem that moire disturbance is generated because the pixel structure pattern of the liquid crystal panel 3 and the vertically striped structure of the lenticular lenses which are constituent elements of the screen 5 in FIG. 1 interfere with each other. Further, as an independent problem, there arises a problem of ghost disturbance caused by the inner round-trip light reflection in a Fresnel sheet used in the screen. The present inventor has found that such Fresnel ghost disturbance occurs terribly particularly at upper and lower ends on the reproduced image screen on the basis of reasons which will be explained later in the detailed description of embodiments.
The present invention which will be disclosed below is configured on the basis of JP-B2-7-19029, U.S. Pat. No. 4,969,751 (JP-A-2-181182), JP-A-5-257114 and JPA-6-250177 filed by the present inventor and further on the basis of a novel idea.